Based on the following three comments by Todd Hollenshead in the article posted some days ago by 3DActionPlanet, I came to the following conclusion:

The comments:

"Now that we've done the multiplayer add-on, we still get a ton of requests for the 'Single Player' Q3 add-on. I think there could be strong demand for it. The asset creation requirements would be significant for an add-on title, and that would be an obstacle."

"The Quake universe is one of the all-time most popular game franchises. A sequel to one of the Quake games is an interesting opportunity under the right circumstances and at the right time. The question would be, does the player fight 'Quake: for Justice', Shub, the Stroggs, or the Vadrigar? At least there's a lot of room to be creative."

"We love nothing better than to make fans happy. From reading the message boards I don't know if they would be more pleased to see something new or to be surprised by the game when we're done with it. And for the record, the name is definitely not "DOOM III". Or at least not yet, anyway. As far as I know."

The conslusion:

Id software will be announcing Quake 4 (not sure for what system), announcing a Quake 3 Singleplayer Addon and showing more of DOOM (this time, from in-game content).

Don't jump to conclusions, little one. I very much doubt id would trust anyone with Q4 (though it would be cool to have a sequel to Q2 with the DOOM engine), and a single-player add-on is even less likely.

Maybe it's wishful thinking, but I think they'll just show more of DOOM, maybe announce the official title (according to Hollenshead NOT "Doom III") and make me cum all over my keyboard.

Well, good news! Can't hardly wait.
They'll probably start working on Quake 4 when Doom 3 is finished. I do hope that they will do another Quake. After all, Quake 3 wasn't that good as everyone expected. I guess they'll try harder on Quake 4.

I hope they'll be showing some serious ingame-footage from Doom 3. We shall see.............

BTW. Only a couple of reply's to important news such as this ---> Yep, this forum is bleeding to death.